Sunday, September 23, 2012

The Land Crabs of Christmas Island

The land crabs of Christmas Island are remarkable for their abundance and the role they play in the ecology of the island's rainforest. Christmas Island is home to twenty-three land or freshwater crabs.

Australia Post issued the fauna stamp set for their territory , Christmas Island, on June 7, 2011 and features the specific species the Land Crabs which available in Christmas Island. The issue comprises of 4 postage stamps which depicted   Red crab (Gecarcoidea natalis),  Robber crab (Birgus latro), Jackson's crab (Karstarma jacksoni), and Blue crab (Discoplax hirtipes) .

Red crab  or Gecarcoidea natalis, is a species of land crab that is endemic to Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean.

Its carapace is up to 116 mm wide, rounded, and encloses the gills. The claws are usually of equal size, unless one becomes injured or detached, in which case the limb will regenerate.The male crabs are generally larger than the females, while adult females have a much broader abdomen (only apparent above 3 years of age) and usually have smaller claws.

Red crabs live in burrows for shelter from the sun.Since they breathe through gills, the possibility of drying out is dangerous. They are famous for their annual migration to the sea to lay their eggs in the ocean.

An exploding population of the yellow crazy ant, an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 10–15 million red crabs (one-quarter to one-third of the total population) in recent years.

Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world. Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August. Coconut crabs  or Birgus latro is the largest land-living arthropod in the world.

The body of the coconut crab is divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right. The next two pairs  are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezer-like chelae at the end.

The coconut crab reaches sexual maturity around five years after hatching.They reach their maximum size only after 40 to 60 years.

Jackson’s crab or Karstarma jacksoni is  a species of genus Karstarma. All species ( 12 species ) in the genus Karstarma are typically found in anchialine pools across the West Pacific.

Anchialine pools are a feature of coastal aquifers which are density stratified, with the water near the surface being fresh or brackish, and saline water intruding from the coast below at some depth.

Karstarma is a genus of karst-dwelling crabs formerly included in Sesarmoides. Karstarma is distinguished from the closely related Sesarmoides by the lack of a stridulatory structure on the cheliped (Legs bearing a chela) which is present in the latter genus.

Blue crab or  Discoplax hirtipes is a species of terrestrial crab and  has a wide distribution across the western Pacific Ocean. It has been recorded from Japan, Taiwan, Palau, eastern Australia, Fiji and the Hawaiian Islands .

Blue crab can be distinguished from other members of the genus Discoplax by the smooth and distinctly inflated carapace. It is also blue or blue-brown in colour, compared to purple or purple-brown in the remaining species.

The breeding season for Discoplax hirtipes lasts seven months, and females must migrate to the sea to release their larvae.

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